This invention relates to filters for removing impurities from molten metal. More particularly, it relates to ceramic foam filters for providing such metal filtration.
Molten metals usually contain solids such as oxides of the metal and other impurities which may cause the final cast product to have undesirable characteristics.
Filters have been devised to remove these impurities from the molten metal during the cast process. Normally these filters are made of ceramic materials so as to withstand the high temperatures associated with molten metals.
The preferred ceramic filters have a foam-like appearance and are referred to in the metal filtration industry as ceramic foam filters. A ceramic foam filter has a plurality of interconnected open cells which trap the impurities there removing the impurities from the molten metal stream.
The fabrication of a typical ceramic foam filter is described in U.S. Pat. No. 4,056,586 issued to Pryor and Gray and assigned to SELEE Corporation, the assignee of the present invention. The Pryor and Gray patent is hereby incorporated herein by reference. In general, a ceramic foam filter is made by impregnating a open cell hydrophilic flexible organic foam material, such as polyurethane, with an aqueous ceramic slurry. The impregnated organic foam is compressed to expel excess slurry. The material is then dried and fired to burn out the organic materials and to sinter the ceramic coating. A solid ceramic foam is thereby formed having a plurality of interconnecting voids having substantially the same structural configuration as the organic foam which was the starting material.
The resulting ceramic filter may be used to filter molten metal such as aluminum, copper, iron, steel or other cast metal. The filter may be placed in an opening in a wall between a molten metal inlet and a molten metal outlet to filter the material. One example of the placement of a filter in a refractory wall is described in U.S. Pat. No. 4,940,489 issued to Cummings and assigned to SELEE Corporation, assignee of the present invention.
It is often desirable, particularly for foundry applications, that the edges of the ceramic foam filter which are to contact the wall be bonded to a solid ceramic coating as a protective layer. The purposes of this protective layer are to enhance mechanical strength, to prevent the passage of metal between the wall and the filter, to prevent penetration of wax which results during an injection molding operation which may be used when the wall is formed with the filter in place, and to reduce the likelihood that the ends of the ceramic foam filter struts will break off during handling.
Several techniques for forming such solid layer are practiced in the ceramic foam industry. For example, it is common to apply a coating to the ceramic foam filter in its wet, green or its fired state by brushing, spraying or troweling on a ceramic slurry with appropriate binders around the edge of the foam filter. An example of forming a solid layer by coating a filter is described in U.S. Pat. No. 4,568,595 issued to Morris.
In addition, fibrous coatings have been glued on or formed by vacuum-forming techniques. Coatings have also been cast in place.
The techniques referred to above suffer from one or more of the following disadvantages:
1. They tend to be costly; PA1 2. They are difficult to automate; PA1 3. The outer surface created tends to be rough and irregular; PA1 4. The coating material tends to penetrate into the pores of the ceramic foam; and PA1 5. Precise control of the coating dimensions is difficult. PA1 A. Forming a ceramic sleeve having an opening therein, with the sleeve having an inner surface; PA1 B. Forming a ceramic foam piece having an outer edge surface, said ceramic foam piece being of a size to fit into the opening of the sleeve; PA1 C. Placing the ceramic foam piece into the opening of the sleeve with the outer edge surface of the ceramic foam piece contacting the inner surface of the sleeve, thereby forming a foam/sleeve composite; PA1 D. Drying and firing the composite.